Field of Invention
This invention relates to electro-optical displays and particularly to intelligent display devices as compact modular components for converting digital data into readable alpha-numeric characters. In particular, the invention relates to a method for fabricating miniature display devices as a double-sided multilayer printed circuit board using polymer thick film technology.
Devices of interest are modular components which convert digital data as from computers to readable alpha-numeric character sets. Complex presentations of words, sentences and even paragraphs can be provided with arrays of display devices while drastically reducing system costs and simplifying system design.
Heretofore arrays of complex display devices could only be employed where space was not at a premium. Relatively simple single layer copper covered and gold plated printed circuit board patterns have been employed to interconnect hybrid circuitry attached to the circuit board. The result was a planar component with a major portion of the display component side of the PC board devoted to support circuitry. As a consequence, complex display circuits could not be readily used in applications where size and spacing, particularly vertical spacing, were critical design parameters. Accordingly, a compact modular display device is needed which can be readily interfaced with a computing device such as a microprocessor and which will permit the display of discrete alpha-numeric characters closely adjacent one another in vertical and horizontal formats.
As used herein, polymer thick film technology relates to low temperature curing epoxy wherein the epoxy cures essentially without reaction with a substrate. By contrast, this technology is to be distinguished from ceramics technology wherein high temperature ink with a glass frit is screened onto a ceramic substrate at such a high temperature that an interaction is induced between the glass frit and the substrate in order to fuse the ink thereon.
The construction of polymer thick film (PTF) overlay circuitry in miniature device applications has created a number of problems including the problems of adherence between the conductor and the dielectric, blistering of the substrate, warping of the substrate, ink bleeding and imperfections in the thickness of the ink, particularly at margins of transition among overlying layers. The consequence has been short circuits between layers and circuit failure particularly where line width and line thickness approach limits imposed by surface tension.
PTF technology enables the overlay of conductors and dielectrics on a printed circuit board by a silkscreen printing process. There are two basic techniques for printing of dielectrics on a substrate. According to the first technique, first the dielectric can be screen printed on the substrate, cured, and reprinted and cured in the same pattern to cover pin holes which generally occur as a consequence of contamination. The print, cure, print process requires that the substrate be removed from the mask for curing and then subsequently be realigned. This process is necessarily slow and requires careful alignment to avoid unacceptable misalignment during the second printing step.
The second known technique for printing dielectrics on a substrate is to apply a pattern and then to reapply the same pattern without removing the substrate and mask from the alignment jig. This technique permits the production of relatively thinner material layers, and it eliminates most alignment problems. However, if a mask contains defects, the consequent impression imperfections remain even after the second application step. The primary problem, however, is bleeding of materials causing poor pattern definition.
The choice of dielectrics and conductors and the curing techniques are critical for achieving proper adherence between the dielectrics, the conductors and the substrate. In addition, the emulsion thickness of the mask and the mesh size of the mask are critical to proper adherence, resolution, and resistance to shock, stress, humidity and temperature.
In the past LED material has been encapsulated on a PC board by a direct casting process in which a high magnification lens was cast directly onto the substrate to cover the LED material die attached to the substrate. However, considerable difficulties were encountered because the mold used in the casting process, typically silicone rubber or polypropylene, deteriorated quickly and required frequent replacement.
Alternatively, high magnification lenses have been formed by injection molding whereby the lenses could be mass produced in a cleaner and easier process overcoming the problems of epoxy casting. However, lenses which have been injection molded do not provide good optical characteristics for viewing the character segments of the LED material when abutted thereto. Specifically, there are frequently voids between the segment material and the lens. The voids have an index of refraction significantly lower than that of the lens. The voids create optical problems, specifically inefficient illumination, distortion and limited viewing angle.